documentation for custom DataSource (#1423)

* documentation for custom DataSource

* satisfy rst linter

* added data_source to index tree

* fixed indention

docs/index.rst

@@ -18,6 +18,7 @@ The ``Doctrine ORM Admin`` provides services to work with the ``Admin Bundle`` a
    reference/templates
    reference/audit
    reference/query_proxy
+   reference/data_source
    reference/troubleshootings
 
 .. toctree::

docs/reference/data_source.rst

@@ -0,0 +1,52 @@
+.. index::
+    double: Reference; Export / DataSource
+
+Export / DataSource
+===================
+
+When using an admins export feature you might want to modify how dates and times are exported.
+This is done by calling ``setDateTimeFormat`` on the data source iterator.
+
+Here's one way to do it:
+
+1. Decorate the default Sonata\DoctrineORMAdminBundle\Exporter\DataSource with your own and call ``setDateTimeFormat`` there.::
+
+      <?php
+      
+      namespace App\Service\Admin;
+      
+      use Sonata\AdminBundle\Datagrid\ProxyQueryInterface;
+      use Sonata\AdminBundle\Exporter\DataSourceInterface;
+      use Sonata\DoctrineORMAdminBundle\Exporter\DataSource;
+      use Sonata\Exporter\Source\DoctrineORMQuerySourceIterator;
+      use Sonata\Exporter\Source\SourceIteratorInterface;
+      
+      class DecoratingDataSource implements DataSourceInterface
+      {
+          private DataSource $dataSource;
+          
+          public function __construct(DataSource $dataSource)
+          {
+              $this->dataSource = $dataSource;
+          }
+          
+          public function createIterator(ProxyQueryInterface $query, array $fields): SourceIteratorInterface
+          {
+              /** @var DoctrineORMQuerySourceIterator $iterator */
+              $iterator = $this->dataSource->createIterator($query, $fields);
+              
+              $iterator->setDateTimeFormat('Y-m-d H:i:s');              
+              
+              return $iterator;
+          }
+      }
+
+
+2. Add the your service in the ``config/services.yaml`` definition.::
+
+      services:
+          ...
+          App\Service\Admin\DecoratingDataSource:
+              decorates: 'sonata.admin.data_source.orm'
+
+